1. Field of the Invention
The present invention relates to a method for reducing output noise of a power amplifier, and more particularly to a method for reducing output power of an output signal of the power amplifier in a frequency band and keeping the output power of the output signal in another frequency band larger than a predetermined value.
2. Description of the Prior Art
In this modern information based society, wireless mobile communication has become an important channel for users to communicate or interchange data with others. For example, users use mobile phones to transmit audio signals so as to communicate or interchange knowledge with other users. Because the communication signals are transmitted with high frequency electromagnetic waves, in order to accurately receive the information carried by the communication signals, the mobile phones must have specific power amplifier to amplify the power of the communication signals. Moreover, the use of a digital signal application instead of an analog signal in wireless telephony technology has been developed, but there are still some limitations due to interference between channels. A Digital Enhanced Cordless Telecommunication (DECT) system, which digitizes the signal and utilizes a Time Division Multiple Access (TDMA) protocol, was defined by the European Telecommunications Standards Institute in 1992 in order to satisfy the increasing service density and quantity requirements.
For example, the Global System for Mobile communication 900 (GSM 900) is based on TDMA, and two frequency bands of 25 MHz each are reserved for it in a mobile unit: 890-915 MHz for transmission and 935-960 MHz for reception. These frequency bands are divided into 124 frequency channels with a spacing of 200 kHz. Moreover, according to the TDMA specification, each frequency channel is divided into 8 time slots. Each mobile phone is given one time slot for transmission and reception, so that each frequency channel can simultaneously carry eight calls and interference between the eight calls in the same frequency channel occurs rarely. However, because the prior art method for controlling the power amplifier is improper, interference between these frequency channels usually occurs.
Please refer to FIG. 1, which is a circuit diagram of a power amplifier 10 according to the prior art. The power amplifier 10 is installed in a mobile phone and is electrically connected to a communication module of the mobile phone. The communication module is used to modulate radio signals into a baseband signal according to the TDMA specification so as to transmit an input signal VRF to the power amplifier 10. The power amplifier 10 is used to amplify the input signal VRF to generate an output signal VOUT. The power amplifier 10 comprises an input terminal 20, an input match circuit 30, a first order circuit 40, a match circuit 50, a second order circuit 60, and an output match circuit 70. The input terminal 20 is electrically connected to the output terminal of the communication module to receive the input signal VRF. The input match circuit 30 is used to match the impedance of the communication module and first order circuit 40. The match circuit 50 is used to match the impedance of the first order circuit 40 and the second order circuit 60, and the output match circuit 70 is used to match the impedance of the second order circuit 60 and an antenna. The first order circuit 40 comprises a first bipolar junction transistor (BJT) 42, which has a base electrically connected to the input terminal 20 via the input match circuit 30 and to a first bias terminal B1 via a first bias resistance 44, a collector electrically connected to the match circuit 50 and to a power supply terminal Vcc via a first collector resistance 48, and an emitter electrically connected to the ground via a first grounded resistance 46. Similarly, the second order circuit 60 comprises a second BJT 62, which has a base electrically connected to the collector of the first BJT 42 via the match circuit 50 and to a second bias terminal B2 via a second bias resistance 64, a collector electrically connected to the output match circuit 70 and to the power supply terminal Vcc via a second collector resistance 68, and an emitter electrically connected to the ground via a second grounded resistance 66.
A bias signal VR is applied to the first bias terminal B1 and the second bias terminal B2 to activate the power amplifier 10 to amplify the input signal VRF. Please refer to FIG. 2, which is a timing diagram of relative signals according to the prior art. The bias signal VB and the output signal VOUT have the same period T. The communication module periodically modulates digital data into the baseband within a given time slot T1. Within each time slot T1, the bias signal VB is pulled up from low to high so that the two BJTs 42 and 62 are turned on. When the two BJTs 42 and 62 are turned on, the power amplifier 10 begins to amplify the input signal VRF to output the amplified output signal VOUT 
If the power that the input signal VRF provides to the power amplifier 10 is defined as an input power PRF, and the power that the output signal VOUT provides is defined as an output power POUT, the output power POUT is capable of being represented as             ∑              i        =        0            n        ⁢                  Ai        ⁡                  (                      P            RF                    )                    i        ,
and is expressed as the equation below:                               P          OUT                =                                            ∑                              i                =                0                            n                        ⁢                                                            A                  i                                ⁡                                  (                                      P                    RF                                    )                                            i                                =                                    A              0                        +                                          A                1                            ⁢                              P                RF                1                                      +                                          A                2                            ⁢                              P                RF                2                                      +            …            +                                          A                n                            ⁢                              P                RF                n                                                                        (        1        )            
where the variable n is an integer greater than 2, and each of the variables A0-An is defined as a power coefficient. One of the power coefficients Aj is defined as a jth power coefficient, where the variable j is an integer. For example, the power coefficient A3 is defined as a third power coefficient. In theory, the output power POUT is absolutely equal to       ∑          i      =      0        n    ⁢            Ai      ⁡              (                  P          RF                )              i  
only when the variable n approaches infinity. However, the power coefficients A0-An are arrangedindescending order, so the output power POUT is usually represented as an approximation, such as       ∑          i      =      0        3    ⁢            Ai      ⁡              (                  P          RF                )              i  
or       ∑          i      =      0        5    ⁢                    Ai        ⁡                  (                      P            RF                    )                    i        .  
Moreover the power coefficients A0-An are not unchanging. The operations of the inner circuit of the power amplifier 10, such as the two BJTs 42 and 62, may influence the power coefficients A0-An.
According to the prior art, when the bias voltage VB is pulled up from low to high, the two BJTs 42 and 62 are turned into an active forward operating mode. However, because of the intermodulation distortion and the non-linearity of the power amplifier 10, the output signal VOUT has many unnecessary noises, which result in an output power Pxe2x80x2OUT provided by the output signal VOUT within an unexpected frequency band larger than a predetermined value. Therefore, the communication within other frequency channels is interfered. Please refer to FIG. 3, which is a spectrum diagram of the output power Pxe2x80x2OUT. A first frequency band I, a second frequency band II, and a third frequency band III are shown in FIG. 3. Each of the three frequency bands has a bandwidth 200 kHz and respectively corresponds to a corresponding frequency channel in the TDMA system. The second frequency band II is used by the communication module, which connects to the input terminal 20, to transmit signals. Both the input signal VRF and the output signal VOUT could be represented as sums of a plurality of sine waves, and the spectrum of the input signal VRF is located within the second frequency band II. If the power amplifier 10 operates ideally, the spectrum of the output signal VOUT should be limited within the second frequency band II. However, because of the intermodulation distortion and the non-linearity of the power amplifier 10, the output signal VOUT has some noises that make the spectrum of the output signal VOUT overlap the first frequency band I and the third frequency band III.
According to the formula (1), the output power Pxe2x80x2OUT and the input power PRF have flowing relationship:       P    OUT    xe2x80x2    =                    ∑                  i          =          0                n            ⁢                        A          n          xe2x80x2                ⁢                              i            ⁡                          (                              P                RF                            )                                i                      =                  A        0        xe2x80x2            +                        A          1          xe2x80x2                ⁢                  P          RF          1                    +                        A          2          xe2x80x2                ⁢                  P          RF          2                    +      …      +                        A          n          xe2x80x2                ⁢                  P          RF          n                    
where the variables Axe2x80x20, Axe2x80x21, Axe2x80x22, . . . , and Axe2x80x2n are the power coefficients of the power amplifier 10 while using the prior art method to control the power amplifier 10.
The previously mentioned intermodulation distortion and the non-linearity of the power amplifier 10 influence the odd order power coefficients Axe2x80x23, Axe2x80x25, Axe2x80x27, . . . , Axe2x80x22m+1 of the power coefficients Axe2x80x20-Axe2x80x2n. However, if the odd order power coefficients Axe2x80x23, Axe2x80x25, Axe2x80x27, . . . , Axe2x80x22m+1 are too large, the output power Pxe2x80x2OUT in some corresponding frequency bands usually exceed a standard value, which result in interference with other frequency bands. FIG. 3 indicates the spectrum of the output power Pxe2x80x2OUT. The spectrum of the output power Pxe2x80x2OUT covers the second frequency band II and extends to the first frequency band I and the third frequency band III, so the communications within the first frequency band I or within the third frequency band III are interfered by the output power Pxe2x80x2OUT. In theory, the output power Pxe2x80x2OUT in the second frequency band II is mainly influenced by the first order power coefficient Axe2x80x21, the output power Pxe2x80x2OUT between a first frequency f1 and a second frequency f2 and between a third frequency f3 and a fourth frequency f4 is influenced by the third order power coefficient Axe2x80x23, and the output power Pxe2x80x2OUT between the second frequency f2 and a fifth frequency f5 and between the fifth frequency f4 and a sixth frequency f6 is influenced by the fifth order power coefficient Axe2x80x25. The larger the first order power coefficient Axe2x80x21, the larger the output power Pxe2x80x2OUT in a fifth frequency band f7-f8. Similarly, the larger the third order power coefficient Axe2x80x23, the larger the output power Pxe2x80x2OUT in the frequency bands f1-f2 and f3-f4. The larger the fifth order power coefficient Axe2x80x25, the larger the output power Pxe2x80x2OUT in the frequency bands f2-f5 and f4-f6.
Because the prior art method applies the same bias voltage VB to the first order circuit 40 and the second order circuit 60, the odd power coefficients Axe2x80x23, Axe2x80x25, Axe2x80x27, . . . , and Axe2x80x22m+1 of the power coefficients Axe2x80x20-Axe2x80x2n can not be effectivelydecreased. Therefore, the communication quality of the mobile phone adapting the prior art method to control the power amplifier 10 is disagreeable.
It is therefore a primary objective of the claimed invention to provide a method for decreasing a plurality of odd power coefficients of a power amplifier by modifying bias voltages that are applied to a first order circuit and a second order circuit so as to adjust thespectrum of the output power of the power amplifier.
The power amplifier is used to amplify an input signal to generate an output signal and comprises an input terminal for inputting the input signal, a first order circuit electrically connected to a first bias terminal and the input terminal, and a second order circuit electrically connected to an output terminal of the first order circuit and a second bias terminal.
The method comprises (a) generating a first bias signal; (b) generating a second bias signal different from the first bias signal; (c) applying the first bias signal to the first bias terminal to control operations of the first order circuit and applying the second bias signal to the second bias terminal to control operations of the second order circuit; and (d) adjusting a waveform of a first bias signal to reduce output power of the output signal in a first frequency band and to keep the output power of the output signal in a fourth frequency band larger than a predetermined value.
These and other objectives and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.